We are working as teachers in a Muslim school, but for some months now our salary has not been paid. We made a decision to strike. Are we allowed to do this, since the school fee is paid per year?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

If this is your situation, then you are allowed to go on strike to get your rights. In fact going on strike is a means and its rule then is the same as the rule of the objective behind it. So, if it is done to get something lawful, then it is lawful and if it is done to get something unlawful then it is unlawful.
Allah knows best.